A switch-mode power converter is a specific type of power converter that produces an output voltage by switching energy storage elements (i.e. inductors and capacitors) into different electrical configurations using a switch network. A switched capacitor power converter is a type of switch-mode power converter that primarily utilizes capacitors to transfer energy. In such converters, also known as switched capacitor circuits, the number of capacitors and switches increases as the transformation ratio or conversion-gain increases. A switched capacitor circuit that has more than one conversion-gain (i.e. mode) is often referred to as a multi-mode switched capacitor circuit
Cascade multipliers are a family of multi-stage switched capacitor power converters that can provide a high conversion-gain using low-voltage transistors. As used herein, conversion-gain represents a voltage gain if the switched capacitor circuit produces an output voltage that is larger than the input voltage or a current gain if the switched capacitor circuit produces an output voltage that is smaller than the input voltage. Energy is transferred from the input to the output by cycling the cascade multiplier network through different topological states. Charge is transferred from the input voltage to the output voltage via a charge transfer path. The number and configuration of the capacitors in each topological state sets the conversion-gain. Therefore, by reconfiguring the cascade multiplier network, the conversion-gain can be modified.
FIGS. 1A-1B illustrate two known reconfigurable cascade multipliers 20A, 20B that receive an input voltage VI from a voltage source 26 and provide an output voltage VO to a load RL. The cascade multiplier 20A is a single-phase asymmetric cascade multiplier that includes a phase voltage P1, diodes D1-D6, pump capacitors C1-C3, and dc capacitors C4-C6. In contrast, the cascade multiplier 20B is a symmetric cascade multiplier that includes phase voltages P1-P2, diodes D11-D14, and pump capacitors C1-C3.
Both of the cascade multipliers 20A, 20B can produce a maximum output voltage VO of four times the input voltage VI. The conversion-gain can be selected based upon first, second, and third enable signals EN1, EN2, EN3. When the first enable signal EN1 is high the output voltage VO is three times the input voltage VI; when the first and second enable signals EN1, EN2 are high the output voltage is two times the input voltage VI; and so on.
Unfortunately, each of the cascade multipliers 20A, 20B require a circuit coupled to the positive terminal of each of the pump capacitors C1-C3 and a circuit coupled to the negative terminal of each of the pump capacitors C1-C3 to change the conversion-gain. The additional circuitry either decreases the efficiency of the cascade multipliers 20A, 20B and/or requires the use of large bypass transistors. Furthermore, the pump capacitors C1-C3 that get bypassed are charged to a voltage that is equal to the input voltage VI minus a diode voltage drop. For example, when the first enable signal EN1 is high the positive terminal of the pump capacitor C1 is biased to a voltage that is one diode drop below the input voltage VI while the negative terminal of the pump capacitor C1 is biased to ground.
When the number of capacitors in the charge transfer path changes, the total amount of charge prior to a reconfiguration event equals the total amount of charge after the reconfiguration event due to charge conservation. This means that the charge in each capacitor prior to the reconfiguration event redistributes among the remaining capacitors upon the reconfiguration event. Furthermore, the voltage across each capacitor before and after the reconfiguration event also changes proportionally to the conversion-gain and the input voltage VI. The polarity of each capacitor voltage change depends on whether the conversion-gain of the switched capacitor circuit is either increasing or decreasing.
A challenge with reconfigurable cascade multipliers lies in the design complexity of operating low-voltage transistors robustly with either a high input voltage range or a high output voltage range. Furthermore, current and/or voltage transients that occur upon each change in the conversion-gain can have significant impact in terms of input electromagnetic interference, transistor operation, and robustness.